I stumbled across this trick. It's new to me, although others may have realized it for a long time.
It works on Webshots, Photobucket, and Flickr in both Safari and Firefox on the Mac.
If you increase the text size of the browser, the photo seems to get larger without degrading in quality. So, depending on the size of your monitor, you can view a much larger picture without having to go to Full Size. If you can still get to the Next button, you can cruise through an album quickly, much more quickly than using slideshow, and the quality is much better. All, while viewing a larger picture.
Then, when you're done, just reduce the text size back to whatever you wish for your normal surfing of the www.